Accurate delivery of one copy of each chromosome in mitosis is essential for faithful transmission of genetic information during each cell division. Errors in this process result in abnormal numbers of chromosomes (a condition described as aneuploidy), which early in development lead to lethal developmental defects and later are hallmarks of human tumor progression. Proper chromosome segregation requires that each chromosome is bi-oriented with one kinetochore attaches to microtubules from one pole of a bipolar spindle while the other sister kinetochore is attached to microtubules from the opposite pole. Improper chromosome attachments, such as synthetic and merotelic attachments, frequently occur in mitosis. A long standing question in the mitosis field is how cells achieve proper stable end-on kinetochore microtubule attachment. Using mammalian cultured cells and purified components in vitro, we intend to determine how formin mDia3 plays a novel role at kinetochores in contribute to stable microtubule attachment. We have shown that knockdown of mDia3 in mammalian cultured cells using siRNA results in defects in metaphase chromosome alignment and stable kinetochore microtubule attachment. These defects can be rescued by a siRNA-resistant wild-type mDia3 construct and mDia3 mutants that are defective in actin nucleation, but not in microtubule stabilization. We have further shown that mDia3 is phosphorylated by Aurora B kinase in vitro and expression of a non- phosphorylatable mDia3 mutant in cells had a chromosome misalignment phenotype. Using microtubule- binding- and EB1-binding-deficient mDia3 mutants for rescue of mDia3 siRNA depletion phenotypes, we will determine whether mDia3 at the kinetochore could act directly by its microtubule activity or indirectly via its interaction partners, EB1 and APC, in contributing to stable kinetochore microtubule attachment. We will use microtubule co-sedimentation analysis with purified components to determine whether stably binding of mDia3, along with EB1 and APC, to the microtubule lattice influences the microtubule binding affinity of the Ndc80 complex, which has been implicated as a core microtubule binding force at the kinetochore. Using immunofluorescence analysis and live cell imaging, we will continue to determine whether Aurora B phosphorylation of mDia3 may represent part of the mechanism for correction of microtubule attachment errors. We will examine how Aurora B phosphorylation affects mDia3 microtubule binding and stabilization activities in vitro using purified components. We will also examine the mitotic phenotypes resulted from the expression of either the non-phosphorylatable or the phosphomimetic mDia3 mutants.